Pyrolysis furnaces are widely used to produce olefins such as ethylene. During the cracking of a hydrocarbon in a pyrolysis furnace, significant quantities of high-boiling hydrocarbons are produced, such as, for example fuel oil, gas oil, and gasoline, as well as lower molecular weight olefin products such as ethylene. The effluent from the furnace, after initial cooling, is introduced to a pyrolysis fractionation unit which removes the heavy end products from the furnace effluent, and recovers heat from the hot effluent stream.
A conventional pyrolysis fractionation unit is illustrated in FIG. 1. Briefly, the pyrolysis fractionation unit includes fractionator 10, fuel oil stripper 12, quench tower 14 and quench drum 16. The partially cooled effluent from the pyrolysis furnace is introduced via line 18 to a lower end of the fractionator 10. A bottoms stream 20 is supplied to the fuel oil stripper 12 where it is stripped by steam introduced via line 22. Steam and hydrocarbon vapor are returned to the bottom of the fractionator 10 via line 24. A fuel oil product 26 is withdrawn from the bottom of the fuel oil stripper 12 via line 26.
Quench oil is circulated from the fractionator 10 via line 28, passed through a series of coolers 30,32 for heat recovery, and returned to the fractionator 10 via respective lines 34,36. Pumps and filters (not shown) are conventionally used in line 28. The coolers 30,32 represent heat exchangers which recover heat for various uses, such as, for example, low pressure steam, dilution steam, plant process use, or the like. A gas oil draw 38 may also be taken from the fractionator 10 and introduced to the fuel oil stripper 12.
Overhead vapor 40 from the fractionator 10 is introduced to the quench tower 14. The vapor is quenched in quench tower 14 by means of water introduced via lines 42, 44 such that an overhead vapor stream 46 is obtained which is at a temperature of about 25.degree.-40.degree. C. Water and condensate from the quench tower 14 are supplied to the quench drum 16 by means of line 48. Water and hydrocarbons are separated in the quench drum 16 to obtain a heavy gasoline stream 50 and a reflux stream 52 which is returned to the top of the fractionator 10. Water is circulated from the quench drum 16 via line 54, cooled in heat exchangers 56,58 and returned to the quench tower 14 by means of lines 42,44 as previously described.
In the operation of this typical pyrolysis fractionation unit, it is desirable to withdraw gas oil draw 38. This reduces the amount of the reflux stream 52 required by the fractionator 10, increasing the amount of heat recovery and the level of heat recovery in exchangers 30,32. Unfortunately, a significant limit on the amount of the gas oil draw 38 is that the viscosity of the circulating quench oil in line 28 significantly increases as the quantity of gas oil draw 38 increases. This increases fouling and pressure drop in the exchangers 30,32.
It would be desirable to be able to lower the viscosity of the circulating quench oil in the pyrolysis fractionator to increase the quantity and level of heat recovery from the feed to the pyrolysis fractionator.